In black-and-white photography, a photon energy of about 4.00 x 10-19 J is needed to bring
about the changes in the silver compounds used in the film. (a) What is the frequency of the
light needed to bring about these changes? (b)
Explain why a red light used in a darkroom does not affect the film during developing.

(a) f = (photon energy)/h
(b) For many B/W films, red light photons do not have enough energy to "expose" the film by changing the silver halide molecules. In any case, they are less able to do so.

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